This invention relates to a gas sensor assembly for gas composition detectors of the type in which a change in the concentration of a gas component such as oxygen, carbon monoxide or hydrocarbon in the exhaust gases discharged from an internal combustion engine is detected as a change in the overall atmosphere.
In recent years, in connection with the control of exhaust emissions from internal combustion engines, gas composition detectors have been used as a means of detecting the air-fuel ratio of the mixture supplied for combustion in the engine.
More specifically, where an exhaust gas purifying catalyst is for example used as a means of controlling the exhaust emissions of an engine, in order that the catalyst may be allowed to perform its function as fully as possible, the air-fuel ratio of mixtures must always be maintained at the proper value and in the case of the carburetor in an ordinary engine or the injection system of a fuel injection type engine, the air-fuel ratio of mixtures tends in fact to vary greatly, even if the carburetor or the injection system is set so as to maintain the air-fuel ratio constant. As a result, in order to maintain the air-fuel ratio constant, it is necessary to detect the actual air-fuel ratio by some means or other and feed the detection signal back to the carburetor or the injection system.
The detection of air-fuel ratio by the gas composition detector is accomplished by directly utilizing the fact that a change in the concentration of any component of the exhaust gases is closely related to the air-fuel ratio of the mixture. In this case, the change in the temperature of the exhaust gases and the concentration change of the gas component occur abruptly and to a great extent and consequently there is a need for an accurate detector from the standpoint of this point.
In one of the known methods of detecting the air-fuel of the mixture supplied to an engine, a transition-metal oxide responsive to an exhaust gas component is used and a change in its electric resistance value is detected.
However, this method is disadvantageous in that since the electric resistance value of the transition-metal oxide varies in dependence on the concentration as well as the temperature of the exhaust gas component, while the air-fuel ratio of mixture can be controlled at the stoichiometric ratio when the temperature of the exhaust gases is held at 850.degree. C., when the exhaust gas temperature is at 350.degree. C. there is the danger of the air-fuel ratio being controlled at a rich ratio as compared with the stoichiometric ratio and thereby failing to control the air-fuel ratio at the stoichiometric ratio, with the result that it is impossible to accurately control the air-fuel ratio unless compensation is made for changes in the electric resistance due to changes in the temperature of the exhaust gases, namely unless temperature compensation is provided.
To overcome this deficiency, gas composition detectors have been proposed as disclosed, for example, in Japanese Laid-Open Patent Application No. 53-136898 and this type of known detector comprises two sintered bodies made of a metal oxide whose electric resistance value varies with the composition and temperature of exhaust gases and only one of the sintered bodies has deposited thereon a catalyst for oxidizing the gas components, whereby the electric resistance value of the catalyst-deposited sintered body is detected while providing the required temperature compensation by the other sintered body having no catalyst, thus detecting variation of the electric resistance value which is dependent only on the gas composition and not affected by the temperature of the exhaust gases and thereby accurately controlling the air-fuel ratio of mixtures.
A disadvantage of this type of known detector is that since the sintered bodies are interconnected with three electrodes which are welded together and the elements are arranged in parallel, the connections of the sintered bodies are not secure with the result that during handling of them the interconnected portion tends to be broken, causing damages to the sintered elements.